An acceleration sensor of this kind is described in U.S. Pat. No. 4,827,091. This known sensor comprises a cylinder made of a conductive material, a magnetized inertial member mounted in the cylinder so as to be movable longitudinally of the cylinder, a conductive member mounted at least on one end surface of the inertial member which is on the side of one longitudinal end of the cylinder, a pair of electrodes disposed at one longitudinal end of the cylinder, and an attracting member disposed near the other longitudinal end of the cylinder. When the conductive member of the magnetized inertial member makes contact with the electrodes, these electrodes are caused to conduct via the conductive member. The attracting member is made of such a magnetic material that the attracting member and the inertial member are magnetically attracted towards each other.
In this acceleration sensor, the magnetized inertial member and the attracting member attract each other. When no or almost no acceleration is applied to the sensor, the inertial member is at rest at the other end in the cylinder.
If a relatively large acceleration acts on this acceleration sensor, the magnetized inertial member moves against the attracting force of the attracting member. During the movement of the inertial member, an electrical current is induced in this cylinder, producing a magnetic force which biases the inertial member in the direction opposite to the direction of movement of the inertial member. Therefore, the magnetized inertial member is braked, so that the speed of the movement is reduced.
When the acceleration is less than a predetermined magnitude,or threshold value, the magnetized inertial member comes to a stop before it reaches the front end of the cylinder. Then, the inertial member is pulled back by the attracting force of the attracting member.
When the acceleration is greater than the predetermined magnitude, or the threshold value, e.g., the vehicle carrying this acceleration sensor collides with an object, the inertial member arrives at one end of the cylinder. At this time, the conductive layer on the front end surface of the inertial member makes contact with both electrodes to electrically connect them with each other. If a voltage has been previously applied between the electrodes, an electrical current flows when a short circuit occurs between them. This electrical current permits detection of collision of the vehicle.
In the prior art, the wall thickness of the cylinder is equal from one end to the other end as shown in FIG. 3.
It was found by the inventors that the inertial member collides with the electrodes with a large speed in the cylinder having the uniform wall thickness, whereby the electrodes vibrate with large amplitude. It causes "chattering " wherein the inertial member and the electrode make repeatedly contact and uncontact with each other, so that a detecting electronic circuit requires various filters to filter noises caused by the chattering.